Frequency bands and modes associated with various protocols are specified per industry standards for cell phone and mobile device applications, WiFi applications, WiMax applications and other wireless communication applications, and the number of specified bands and modes is increasing as the demand pushes. Examples of the frequency bands and modes for cell phone and mobile device applications are: the cellular band (824-960 MHz) which includes two bands, CDMA (824-894 MHz) and GSM (880-960 MHz) bands; and the PCS/DCS band (1710-2170 MHz) which includes three bands, DCS (1710-1880 MHz), PCS (1850-1990 MHz) and AWS/WCDMA (1920-2170 MHz) bands. Examples for uplink for transmit (Tx) signals include the frequency ranges of DCS (1710-1785 MHz) and PCS (1850-1910 MHz). Examples for downlink for receive (Rx) signals include the frequency ranges of DCS (1805-1880 MHz) and PCS (1930-1990 MHz). Examples of frequency bands for WiFi applications include two bands: one ranging from 2.4 to 2.48 GHz, and the other ranging from 5.15 GHz to 5.835 GHz. The frequency bands for WiMax applications involve three bands: 2.3-2.4 GHz, 2.5-2.7 GHZ, and 3.5-3.8 GHz. Use of frequency bands and modes is regulated worldwide and varies from country to country. For example, for uplink, Japan uses CDMA (915-925 MHz) and South Korea uses CDMA (1750-1780 MHz). In this document, “modes” refer to WiFi, WiMax, LTE, WCDMA, CDMA, CDMA2000, GSM, DCS, PCS and so on; and “bands” or “frequency bands” refer to frequency ranges (700-900 MHz), (1.7-2 GHz), (2.4-2.6 GHz), (4.8-5 GHz), and so on. Laptops, tablets, personal digital assistants, cellular phones, smart phones and other mobile devices include a communication system which is generally designed to have paths or chains to process signals in multiple modes and bands. In this document, a combination of a mode and a band is denoted as “mode/band.”
As new generations of wireless communication devices become smaller and packed with more multi-mode multi-band functions, designing new types of antennas and associated air interface circuits is becoming increasingly important. In particular, signals in different frequency bands in such a dense RF circuitry tend to interfere with each other, and in some cases even damage sensitive components if the gap between the bands is small. Therefore, a proper isolation provision needs to be considered in designing such a multi-mode multi-band system having multiple paths therein.